Is Diesel A Greener Option For Cars Than Gasoline?

Dear EarthTalk: Are there any environmental benefits to diesel-powered cars?

—Bill Darcy, Concord, NH

The high fuel economy of a diesel-powered vehicle, such as the Volkswagen Golf GL TDI, might seem like a no-brainer choice for anyone looking for a new car that will consume less gas and money. But although the TDI gets far better gas mileage than a similar gasoline-powered Golf, its diesel engine emits considerably more harmful pollutants into the air.

In fact, a Swedish study found that diesel-powered cars in India had twice the cancer potency level of gasoline-powered vehicles in that country, results supported by German research. And the Union of Concerned Scientists (UCS) reports that, in California, diesel exhaust accounts for 70 percent of the cancer risk from that state’s polluted air. According to the U.S. Environmental Protection Agency (EPA), the particulates, or soot, in diesel exhaust cause a host of health problems, including: irritation of the eyes, nose and throat; heartburn; headaches and lightheadedness; and asthma and lung disease.

The EPA’s Green Vehicle Guide rates cars and trucks on a scale of zero to 10 (10 being the best) based on their emissions of smog-forming pollutants per every 15,000 miles driven. While the gasoline-powered Golf received a rating of seven, emitting just under 12 pounds of pollutants, the Golf TDI got a one, spewing out 44 to 59 pounds over the same distance driven. The gargantuan Ford Excursion and Hummer H2—with gas mileage in the single digits—each received just a two.

Some diesel proponents claim that the increased fuel efficiency of diesel engines could significantly reduce the carbon dioxide emissions responsible for global warming. But, according to UCS, widespread adoption of diesel-powered cars would yield less than a five percent reduction in carbon emissions, while significantly increasing air quality problems.

Under current EPA regulations, diesel vehicles are allowed to emit 16 times more particulates into the air than gas vehicles. However, California is taking the lead in phasing out loopholes that allow diesel cars to out-pollute their gas-powered equivalents, and other states are sure to follow. While pollution from diesel cars has been cut by 80 to 90 percent over the past two decades, it must be reduced another 75 to 90 percent within the next few years just to meet pollution standards in California. Additionally, the federal government recently adopted new “Tier Two” tailpipe standards that will require significant cuts in diesel vehicle emissions nationally.

Drivers looking to minimize their pollution impact while maximizing fuel efficiency would be smarter to ignore diesel offerings and go for a gasoline-electric hybrid like the Toyota Prius or Honda Insight. And once auto makers can resolve refueling infrastructure issues, zero-emission hydrogen-powered fuel cell cars should provide another cost-efficient and environmentally friendly alternative to the gas-guzzlers dominating the road today.